Radiation is frequently used to treat cancer tumors. For treating localized cancers such as tumors, the goal is to maximize the radiation level at the tumor and minimize radiation damage to the rest of the body. This is achieved by irradiating the tumor with a narrow beam of radiation aimed at the tumor from many different angles so as to maximize the radiation at the tumor while sparing surrounding healthy tissue.
Prior to radiation treatment, the patient will usually receive a computed tomography (CT) scan to diagnose and locate the tumor and also to provide the anatomical information necessary to develop a treatment plan. A treatment plan consists of a series of positions for the radiation therapy source relative to the patient that will produce the desired radiation distribution centered on the tumor in the patient. Each position of the radiation therapy source may have different radiation energy levels, durations, and control of the profile of the radiation therapy beam.
It is critically important that the location of the tumor be accurately known so that the planned radiation distribution can be aligned with the tumor. If the radiation distribution is not accurately aligned with the tumor, the tumor will not receive a sufficient radiation level to damage or kill the tumor and healthy organs may receive damaging levels of radiation.
Various methods are used to image the tumor shortly before the radiation therapy source is activated.
These methods have several limitations. The image quality may be insufficient to show the tumor and its location and/or the imaging procedure may be sufficiently slow that there can be no assurance that the tumor is in its expected position when the radiation therapy source is activated some time later. The tumor may have moved due to motion of the patient or due to motion of the tumor within the body due to respiration, heartbeat, and/or peristalsis.
A radiation treatment system may have a linear accelerator radiation source and an x-ray imaging system consisting of an x-ray source and a large-area x-ray detector. These are attached to a rotating mechanical gantry. By rotating the gantry around the patient, many two-dimensional x-ray projection views through the patient can be obtained and a three-dimensional cone-beam CT image can be reconstructed showing the tumor and other anatomical landmarks.
Since the linear accelerator, x-ray source, and large-area detector are fixed to the same mechanical gantry, the location of the tumor relative to the radiation therapy beam can be readily and accurately determined. If the tumor is not in the correct position with respect to the radiation therapy beam, the table that supports the patient can be repositioned. Alternately, the linear accelerator energy pattern can be adjusted so that the modified radiation energy pattern is aligned with the tumor.
The x-ray source and large-area detector are arranged approximately at right angles to the radiation therapy beam. This is done to avoid direct radiation from the linear accelerator striking these components, which can be damaged by the high radiation levels from the linear accelerator.
This type of imaging system cannot provide the rapid imaging necessary to verify that the tumor is in the radiation therapy beam at the time of treatment. The rotation of the gantry around the patient to acquire the many two-dimensional projection images and the subsequent CT reconstruction typically takes many tens of seconds. CT has high contrast resolution and therefore usually has sufficient resolution to allow the tumor to be visualized. A single two-dimensional view has lower contrast resolution and may be unable to image the tumor with sufficient resolution to determine its location. Further, if the tumor moves approximately along the direction of the x-ray beam, this motion will likely not be detected in the image.
A multi-axis robotic positioning system can control the position of a linear accelerator and the direction of its radiation therapy beam.
Two x-ray imaging systems, each consisting of an x-ray source and a large area detector, can be arranged so that the patient is imaged from two approximately orthogonal views. This provides stereotactic views of the patient so that the location of the tumor can be determined from these two views and a previously acquired CT image.
The multi-axis robotic positioning system can adjust the position and/or direction of the radiation therapy beam so that the desired radiation pattern is aligned with tumor in the patient.
This stereotactic imaging system has several problems. The tumor may not be visible in one or both of the views. The contrast resolution may not be sufficient or the tumor may be obstructed by certain anatomy within the patient. The radiation therapy source may be between the source and detector, blocking one of the views.
What is needed is an imaging system capable of producing rapid high quality images in order to guide radiation therapy. Furthermore, the system should provide low radiation imaging.